I have done it. The best way to tell is to create a .xls file (not a .xlsx) and then save it as an html file.
Then look at the source of the html file. You will see some css classes at the top, and then if you look at the data below, you will see that they apply to the sheet.
So just a little reverse engineering ...
FYI - if you try to open it in 2007 or later, you will get an initial warning, but then everything will work fine.
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